Well I was on the phone with Web Services for over an hour today. I finally convinced them that this is a bug after sending multiple screenshots. They have made a ticket to get a fix in the works, but who knows what priority this will get.

For now, I've found my preferred workaround is to add my husband as my travel companion but to leave his FF# blank so that I can add his KTN there. This means that his AAdvantage number, DOB, and place of residence won't populate automatically when I book for two people, but his KTN will; we'll see if it's easier for me to enter those memorized facts about him than the KTN which I most certainly won't be able to memorize accurately.